Used foundry sand is regenerated to remove such foreign matter as bentonite, synthetic resin, particles of coal and coke, etc. and convey them away pneumatically. These substances are either purposely added or occur on their own during casting. The regenerated sand is as good as new and can even be employed in cores. The process is carried out in a regenerator with a downstream separator. Dust released in the regenerator is extracted continuously or discontinuously with at last one current of air and transferred to the separator, where it is intercepted and precipitated. The dust can be precipitated altogether or class by class, depending on the type of separator.
Known regenerators operate in accordance with various principles. Some include rotating drums. The drums may accommodate turbulators or similar components. Others feature mechanical beaters, centrifugal scrubbers, fluidizers, and stationary abraders. The separators are usually cyclones with fine-dust filters downstream.
One substance that is particularly difficult to remove is bentonite. It can account for 30% and even more of the mass of the used sand. Bentonite is present in different states that require different strategies to remove. The substance occurs in recirculating molding sands in iron-and-steel foundries in two forms, active and hard. Active bentonite is needed to bind the sand in the mold. Hard bentonite on the other hand is ballast material. The heat of the molten metal destroys its binding power, and it usually bakes in the form of a shell around each grain of sand. This process is often called ooliting. Hard bentonite shells must be either cracked open by knock or impact in order to regenerate the sand or ground by powerfully abrading the particles together. In the latter approach the edges of the particles will simultaneously be rounded off, which is an advantage.
Removing the residual active and refractory bentonite, coal dust, and core binder during regeneration overtaxes many regenerators. The sand will either take too long to regenerate or will be left so contaminated as to be useless. It has been demonstrated that the synthetic-resin binders employed in most contemporary cold-box or hot-box processes must include essentially less than 0.5% residual dust by mass. Otherwise, the regenerated sand must be refreshed with a lot of new sand, and too much resin will have to be added. These techniques need to be eliminated as much as possible for economical and environmental reasons.
Research demonstrates that the composition of used sand can extensively change so rapidly that even the parameters of a regenerator at the same foundry will frequently have to be revised. The revisions, however, may be wrong if the precise situation is not known. It has for example been discovered that the composition of used sand obtained at different times from the same point in the same foundry can differ so extensively that some can take as little as 20 minutes and some as much as 80 minutes to regenerate satisfactorily. Still, foundries lack testing facilities that will provide the necessary data rapidly enough to appropriately tailor a regeneration in process. Regeneration in this case is accordingly conducted for 30 minutes with abrasion force and dust removal maintained at constant levels. The result is poor regeneration, unnecessary energy consumption, and low output.
Foundry sands differ from batch to batch due to their varying ratios of mold sand to core sand and in accordance with how much heat they take. Requirements, however, will also vary while one and the same charge is being regenerated. Much of the failure that occurs in the regeneration of foundry sands derives from setting such parameters as machinery-operating times, processing rates, impact and abrading forces, and dust-removal rates at unvarying averages. The consequences are uneconomical operation and poor regeneration accompanied by defective cores and castings. Another aggravation is that current machinery is not designed or manufactured to individually or interactively adjust to necessary momentary variations in the abrading and dust-removal situation.